A gas burner is the fire producing device used in a warm air furnace, a heat exchanger, a boiler, an oven, and the like. Typically, the gas burner controls the flow rate and mixing of air and gas and includes the controls that do the ignition and safety monitoring of the flame. For many applications of a gas burner, the amount of heat required is not constant. The amount of heat required may vary according to the weather, the process load, and other conditions. To deal with varying loads, banks of multiple burners have been used. The banks of multiple burners may be sequenced to produce the required amount of heat. Alternatively, a burner with a variable firing rate may also be used. A burner with a variable firing rate can be a staged burner, capable of operating either at a low fire or high fire, or it can be a modulating burner. A modulating burner is capable of being controlled to operate at any firing rate within a range between its minimum and maximum firing rates. That range is typically 50%-100%, with some of the better burners being capable of 33%-100%. That means that when the heat requirement is less than the minimum firing rate of the burner, 33% in the case of the better burners, the only alternative is to periodically cycle the burner on and off at the minimum rate in order to produce a lesser amount of heat than is produced at the minimum rate. Unfortunately, this results in fluctuating temperatures and therefore less than ideal control when operating in this mode.
Accurate and consistent temperature control is improved if a burner is capable of operating at a very low minimum firing rate. A great deal of effort in the industry has been expended toward achieving the goal of having a very low minimum firing rate. Typically, efforts at providing such capability have concentrated on control of the gas flow and control of the secondary air.
With respect to control of the gas flow, the maximum fire rate of a gas burner is typically controlled by the sizing of the main gas orifice. The size is typically set when the burner is manufactured and is invariable thereafter. The maximum firing rate occurs when a specified gas pressure is present at the fixed orifice. To effect the minimum firing rate on a modulating gas burner, it is common practice to control a butterfly gas valve or other similar device to cause a reduction in the gas pressure to the fixed orifice. Reducing the gas pressure causes a reduction in the gas flow rate through the fixed orifice, thereby reducing the firing rate of the burner. Typically, a control actuator is mechanically linked to the butterfly gas valve to also control a combustion air damper, such that both the gas and the combustion air are simultaneously reduced to achieve the minimum firing rate. Alternatively, the combustion air damper only is controlled. Such control reduces the air pressure within the burner. A suitable pressure regulator is then used to sense the reduced air pressure and to control the gas pressure proportionately.
Because the flow rate to a fixed orifice varies as the square of the pressure across it, there are practical limits as to how low the flow can be reduced using either of the foregoing techniques. As an example, if the burner utilizes 4.0 inches water column orifice pressure at the maximum firing rate, the pressure would have to be reduced to unmanageably low levels to operate in the region below approximately 20% of the maximum firing rate. Such levels are indicated in Table 1 below.
100% 4.00 In. W.C. 50% 1.00 In. W.C. 33% .44 In. W.C. 20% .16 In. W.C. 10% .04 In. W.C. 5% .01 In. W.C.
As indicated above, secondary air may be also controlled to achieve a minimum firing rate. Secondary air is that air which is introduced directly into the combustion zone. Typically the combustion air to a modulating gas burner is controlled by a pivoting damper blade. A pivoting damper blade is inadequate for a burner that is going to be modulated down to a minimum firing rate that is less than 25% of the maximum firing rate. A pivoting damper blade simply does not allow precise enough control near and at the desired minimum firing rate.
On gas burners that control secondary air to proportion combustion air, primary air is not presently varied in any fashion in order to affect the minimum and maximum burning rates. Primary air is that air that is mixed directly with the gas stream before it enters the combustion zone. Having a source of primary air is common practice with many types of gas burners.
As previously indicated, there is a need in the industry for a gas burner that is capable of operating efficiently at very low minimum firing rate. Such firing rate should be in the range of less than 25% of the maximum firing rate. In order to achieve such a low minimum firing rate, a new means of accurate and consistent temperature control is required.